1. Field of the Invention
The field of the invention is related to the visual displays and in particular to a dual mode active/passive display.
2. Prior Art
Displays based on the colloidal and electrochromic light valve principal are generally well known in the art. Colloidal light valves as shown by Land in U.S. Pat. Nos. 1,955,923, issued Apr. 23, 1934 and 1,963,496, issued June 19, 1934, embody dichroic dipole particles suspended in a colloid or fluid medium in which the randomly oriented dichroic dipole particles can be caused to align under the influence of electrostatic or electromagnetic fields. The dichroic particles may be reflective or opaque when randomly disposed and transparent when aligned under the influence of an appropriate magnetic or electrostatic field. Electrochromic light valves, such as shown by Castellion in U.S. Pat. No. 3,807,832, issued Apr. 30, 1974 exhibit coloration and bleaching in response to the presence or absence of an electrical field. Although the mechanisms of colloidal and electrochromic light valves are different, their basic functions are equivalent.
Active displays are made by backlighting a colloid or electrochromic light valve. The basic structure of a typical colloidal light valve is shown by Marks in U.S. Pat. No. 3,512,876, issued May 19, 1970. In the unactivated state, the random orientation of the dichroic particles absorb incident light and cause a relatively large transmission loss through the valve. When an AC field is applied across opposing transparent electrodes deposited on the inner surfaces of the substrates, the dichroic dipole particles align along the lines of the field and the colloid suspension becomes transparent. Illumination from a light source behind the light valve passes through the portion of the colloid suspension where the particles are aligned and a light image corresponding to the activated electrode pattern is visible through the front window of the display. Brownian movement of the suspended dichroic particles after the removal of the field will cause the particles to assume their prior light absorbing random orientation.
Passive display panels are made by applying a reflective coating on the inner surface of the rear substrate. This reflective coating may be a mirror surface as taught by Castellion in U.S. Pat. No. 3,807,832, cited above, or may be a reflective surface having a color different from the color of the colloid suspension in the unactivated state. In the unactivated state, the reflective surface is occluded by the dichroic particles and only the color of the dichroic particles is visible when the display is viewed from the same general direction of the incident light. When an AC field is applied across transparent electrodes disposed on the inner surfaces of the substrates, the dichroic particles align with the field and the area underlying the electrodes becomes transparent causing the reflective surface to be illuminated by the incident light producing a visible image at the front face of the display panel.
Active displays work well under relatively low incident light levels but have a tendency to wash out at high light levels, while passive displays work well at high incident light levels but lose visibility under low light levels.
Dual mode displays which are operative in both the active and passive mode are known in the liquid crystal art in which the reflective surface is made from a semi-transparent material. In liquid crystal displays, the difference between the optical density of the liquid crystal material in the activated and non-activated state is sufficient to permit high contrast images in both the active and passive modes of operation. In displays using colloidal suspensions of dipole particles or electrochromic materials, the differences in optical density between the activated and non-activated state tends to be somewhat less such that when the optical density of the electro-responsive material is optimized for use in the passive mode, the optical density of the material is insufficient to produce a high contrast image in the active mode and vice versa. This is because in the passive mode, the light passes through the electro-responsive material twice while in the active mode, the light only passes through the material once. Dual mode displays using a single cell light valve in which the optical density of the electro-responsive material is a compromise between the two modes of operation have had less than desirable image contrast.
The disclosed invention is a dual mode display embodying two tandemly arranged light valves in which the optical density of the electro-responsive material in each light valve is selected to optimize the image contrast in both modes of operation.